Pulse generating circuit



Dec. 2, 1947. MAXWELL 2,431,952

PULSE GENERATING CIRCUIT Filed June 7, 1944 SOURCE or PULSE. van/165 Fig.2. l Fig.5.

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- DonaklE.MaxwdL Patented Dec. 2, 1947 PULSE GENERATING CIRCUIT Donald E. Maxwell, Syracuse, N. Y., assignor to General Electric Company, a corporation of New York Application June 7, 1944, Serial No. 539,221

7 Claims.

My invention relates to electric pulse generating circuits, and more particularly to means for shaping the trailing edge of a pulse to provide a more nearly rectangular pulse wave form.

Certain radio communication and detection circuits in current use employ pulse modulated rather than continuous carrier waves for transmission. The modulating pulses are of very short duration and preferably of substantially rectangular wave shape. The pulsed carrier wave may be utilized in various ways, such as by transmission and reflection of a series of pulses for object detection, or by signal modulation of the pulse width, amplitude, or frequency to convey intelligence.

Whenever rectangular pulses of voltage are applied to circuits of an inductive nature, there arises the problem of dissipating the stored energy of the inductive circuit at the end of the pulse. This problem is commonly encountered when the output of a pulse generator is utilized to modulate the high frequency oscillations of a magnetron oscillator. In such a circuit a pulse transformer is frequently interposed between the pulse generator and the magnetron oscillator tube both to increase the voltage applied to the magnetron tube and to provide a charging path for the capacitive pulse forming element. The transformer and magnetron oscillator tube provide a certain amount of distributed and stray capacitance to ground which forms with at least a portion of the pulse transformer a resonant circuit in which the energy stored in the inductive transformer winding has a tendency to initiate transient oscillations at the termination of each pulse. The pulse transformer itself also includes a certain amount of internal leakage reactance and distributed capacitance having a tendency to set up additional oscillations of very much higher frequency at the termination of each pulse period. The oscillations due to the stray ground capacitance are high in frequency relative to the usual pulse repetition rate but of low frequency relative to the oscillations arising from the transformer interval circuit constants.

The high frequency oscillations are particularly objectionable for a number of reasons. One reason is that the inverse voltage peaks produced by the oscillations add to the charging voltage of the pulse forming element, thereby tending to produce arc-over of the pulse triggering tube. Another objectionable effect of the high frequency trailing edge oscillations is that the voltage peaks of polarity similar to that of the desired pulse tend to start oscillation of the magnetron tube in undesired spurious modes. By proper transformer design, it is possible to keep the lower frequency oscillations within permissible limits.

Accordingly, it is an object of my invention to provide new and improved means for shaping the trailing edges of voltage pulses to provide a more nearly rectangular pulse wave shape.

It is a further object of my invention to provide new and improved means for suppressing undesired high frequency oscillations at the trailing edges of electric pulses.

It is a still further and more specific object of my invention to provide new and improved means for dissipating stored energy in a pulse load circuit thereby to suppress high frequency oscillations at the trailing edge of each pulse.

In accordance with my invention, the above and other objects are attained by connecting in parallel circuit relation with the inductive portion of the pulse load circuit an energy dissipating circuit comprising a resistive impedance connected in series circuit relation with a blocking capacitor. The resistor should be non-inductive and comparable in impedance to the shunted inductance. The capacitance of the capacitor should be relatively small, so that it presents only a very small impedance to high frequency oscillation current, but presents a substantial impedance to currents of pulse frequency.

My invention will be more fully understood and its objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawing, in which Fig. 1 is a schematic circuit diagram of a pulse generating circuit embodying my invention, and Figs. 2, 3, and 4 are graphical representations of pulse Wave shapes illustrating the operation and effect of my invention.

Referring now to the drawing, and particularly to Fig. 1, I have shown a pulse generating circuit comprising a source I of substantially rectangular recurrent voltage pulses. The succession of pulses originating in the source I is supplied, through a transformer 2, to the input circuit of an electron discharge device 3. The discharge device 3 comprises an anode 4, a cathode 5, and a control electrode 6, and is connected as a trigger switch in the output circuit of a pulseforming capacitive energy storing element, shown by way of example as a condenser 7. The electron discharge device or pulse triggering tube 3 is periodically rendered conductive by positive voltage pulses from the source I, thereby to connect the condenser l for pulse discharge through an output circuit including an autotransformer 9 and a magnetron oscillator tube 8 having an anode 8a and a cathode so. As shown in the drawing, the magnetron oscillator tube 8 is connected across the secondary winding of the autotransformer 9, the primary winding ii] of which is connected in parallel circuit relation with the pulse discharge path through the magnetron oscillator 8. The primar winding H! of the autotransformer 9 serves as a direct current path for recharging the condenser 1 at the termination of each pulse. The condenser charging circuit also includes a high resistance H and a source of unidirectional electric current supply, such as the battery l2, connected in series circuit relation between the anode and cathode of the electron discharge device 3. The cathode of the discharge device 3 and the anode 8a of the magnetron oscillator 8, as well as the negative terminal of the battery 12 and one terminal of the autotransformer 9, are connected together and grounded, as shown at Fig. 1.

The input circuit of the pulse triggering tube 3 comprises a secondary winding i3 of the input transformer 2 connected between the cathode 5 and control electrode 6 of the discharge device 3 through a suitable coupling capacitor Hi. The discharge device 3 is negatively biased to cut-off by any suitable means, such as for example, a battery 15 and a grid resistor 16 connected between the cathode 5 and control electrode B.

It is found in practice that the autotransformer 9 and the magnetron oscillator tube 8 have a certain amount of stray and distributed capacitance to ground. This capacitance is indicated by dotted lines at l8 in Fig 1 between the anode and cathode of the magnetron 8. The stray and distributed capacities represented by the capacitor l8 form with the autotransformer 9 a resonant circuit having a natural frequency of oscillation relatively high with re spect to the repetition rate of the pulses from the source I. This natural oscillation frequency is ordinarily of the order of 100 kilocycles per second and is of relatively small amplitude with a properly designed transformer. The pulse repetition rate, on the other hand, is ordinarily of the order of 500 to 5000 pulses per second in radio detection apparatus or pulse communica tion apparatus. The autotransformer 9 has also a certain amount of internal leakage reactance and distributed capacitance. In pulse transformers of the type commonly employed in the generation of pulses of the character described these internal circuit constants tend to initiate a very high frequency trailing edge oscillation superposed upon the 100 kilocycle oscillation described above. These superposed high frequency oscillations are ordinarily of the order of '7 to 10 megacycles per second.

For the purpose of suppressing high frequency oscillations at the termination of each pulse due to internal resonance of the leakage inductance and distributed capacitance within the transformer, I have provided across the primary winding H! of the autotransformer 9 an energy dissipating circuit comprising a noninductive resistor l9 and a blocking capacitor 20 connected in series circuit relation. The condenser 20 is of very small capacitance in order to provide a very large impedance to pulse frequency currents. The capacitor 20 therefore acts substantially to block the passage of pulse frequency currents through the resistor l9, so that no appreciable power at pulse frequency is absorbed in the resistor. If the condenser 20 is sufficiently large to permit appreciable loss of pulse power in the resistor 19, the rectangular pulse shape is impaired. The resistor l9 itself should be non-inductive and of a resistance comparable in magnitude with the impedance of the primary winding ill of the autotransformer 9. It has been found in practice that satisfactory damping of trailing edge oscillations will be effected without adversely affecting pulse shape if the time constant of the R. C. circuit I9, 20 is very much smaller than the pulse period. For example, in a practicable embodiment of the invention, a damping circuit i9 and 20 having a time constant of .0225 microseconds provided very effective damping of trailing edge oscillations for pulses having a width or period within a range of 0.5 to 2.5 microseconds.

In view of the foregoing detailed description of the function and arrangement of the various elements of the pulse generating circuit shown at Fig. 1, the mode of operation of the circuit as a whole will be understood from the following brief description. As previously mentioned, the electron discharge device 3 is biased to cutoff by the battery 15. When the discharge device 3 is not conducting, the condenser 1 is maintained charged to substantially the full voltage of the battery l2 through the charging circuit comprising the resistor H, the condenser I, and the primary winding l0 of the autotransformer 9. The voltage pulses applied from the source I through the transformer 2 produce recurrent and periodic positive potentials upon the control electrode 6 for predetermined pulse intervals. thereby to render the discharge device 3 conductive. When the discharge device 3 becomes conductive, the condenser 1 discharges through a load circuit comprising the discharge device 3, the magnetron oscillator tube 8, and the upper portion of the autotransformer 9. The voltage drop through the discharge device 3 is relatively small so that substantially the full voltage of the condenser l is impressed across autotransformer primary winding l0. Through the transformer secondary winding a greater voltage is impressed upon the tube 8.

It will be noted that the pulse load circuit includes the transformer primary winding H1 in parallel with the discharge path through the magnetron tube 8. However, since the autotransformer primary winding i0 is highly inductive, very little magnetizing current flows through the primary winding during the pulse period. During the conductive period, however, a small amount of current is gradually built up in the transformer winding i0, so that at the termination of the pulse a certain amount of energy is stored in the transformer. At the termination of the pulse discharge this stored energy initiates both a relatively low frequency oscillatory charge and discharge of the distributed and stray. capacitance represented by the condenser 18 and the high frequency oscillations characteristic of the internal transformer circuit constants. The oscillations thus produced cause a fluctuation of voltage across the. autotransformer 9. I have shown at Figs. 2 and 3 the manner in which the voltage across the autotransformer primary and secondary windings, respectively, varies during and after a pulse discharge in the event that the energy Memos dissipating circuit [9, is not. provided. It will be noted from Fig. 2 that the voltage across the transformer primary winding I 0 fluctuates much more violently than that across the secondary winding and that inverse voltages of appreciable intensity appear across the winding l0 and the discharge device 3.

In accordance with my invention the violent trailing edge voltage oscillations shown at Figs. 2 and 3 are eliminated by the energy dissipating resistor I9 and the capacitor 2e. At Fig. 4. I have shown a pulse wave shape diagram illustrating the manner of variation of voltage across the transformer secondary winding and, hence, across the magnetron oscillator tube 8 when the resistor l 9 and capacitor 28 are provided tc absorb th energy stored in the auto transformer 9 at the termination of a pulse. It will be evident by comparison of Figs. 3 and a that the duration and intensity of trailing edge oscillations are greatly reduced by the series network I9, 29 without impairing the shape of the desired pulse itself.

While I have described only one preferred embodiment of my invention by way of illustration, many modifications will occur to those skilled in the art and I therefore wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a capacitive storage ele-.

ment, a load circuit, means for establishing periodic pulse discharges of said capacitive stor age element through said load circuit including switching means and means for periodically rendering said switching means conductive for predetermined pulse intervals, means including an autotransiormer nected in shunt circuit relation to said load circuit for charging said capacitive storage element between said pulse intervals, said autotransformer in conjunction with stray circuit capacities forming a resonant circuit having a tendency to initiate at the termination of each pulse discharge electric oscillations at a natural frequency appreciably greater than the periodicity of said pulses, and means for damping said oscillations comprising resistive energy dissipating means connected across said primary winding and capacitive means for rendering said energy dissipating means substantially ineffective for pulse frequency currents whereby the shape of said pulse is not impaired.

2. In combination, a capacitive storage element, a load circuit including a unilateral conducting device, means for establishing periodic pulse discharge of said capacitive storage element through said load circuit including an electron discharge device having a control electrode, means including said control electrode for periodically rendering said discharge device conductive for predetermined pulse intervals, means for charging said capacitive storage element between said pulse intervals comprising an autotransformer having electric current supply, said autotransformer providing with stray circuit capacities a resonant circuit having a natural frequency appreciably greater than the periodicity of said pulses and having a tendency initiate electric oscillations at said frequency at the termination of each pulse discharge, and means for damping said electric oscillations comprising a resistor and a capacitor having a primary winding con connected in series circuit relation across said inductive impedance.

3. In a pulse generating circuit, a capacitive storage element, a discharge circuit for said capacitive element comprising an electron discharge device and a unilaterally conductive load element, means for establishing successive Pulse discharges of said capacitive element through said load circuit including means for re currently rendering said electron discharge device conductive for predetermined pulse intervals, means for recharging said capacitive element between said pulse intervals comprising an auto-- transformer having a primary winding connected in parallel circuit relation with said load element, and means for damping trailing oscillations initiated by said primary winding at the termination of each of said pulses comprising a resistance and capacitive in series connected in parallel circuit relation with said primary winding.

4. In a pulse generating circuit, a capacitive storage element, a discharge circuit for said capacitive element comprising an electron discharge device and a unilaterally conductive load element, means for establishing periodic pulse discharge of said capacitive element through said load element comprising means for periodically rendering said discharge device conductive for predetermined pulse intervals, means for recharging said capacitive storage element between said pulse intervals comprising an inductive impedance connected in parallel circuit relation with said load element, said inductive impedance forming with stray circuit capacities a resonant circuit having a tendency to initiate at the termination of each pulse discharge electric oscillations at a frequency appreciably greater than the periodicity of said pulses, and means for damping said oscillations comprising a resistor and a blocking capacitor connected in series circuit relation across said inductive impedance, said blocking capacitor having a low impedance for oscillations at said resonant frequency and a high impedance to pulse frequency currents,

5. In combination, an electron discharge device including an anode, a cathode, and a control electrode, means for supplying to said control electrode a succession of voltage pulses arranged recurrently to render said discharge device conductive for predetermined pulse intervals, a capacitive pulse-formingenergy storage element, a load circuit connected to said capacitive storage element through said discharge device, said load circuit including an autotransformer having a primary winding, and means including a resistor and capacitor in series, having a time constant very much smaller than the pulse period, connected in parallel circuit relation with said primary winding to dissipate the energy stored in said inductive element at the termination of each pulse discharge of said capacitive element through said load circuit.

6. In combination, an electron discharge device including an anode, a cathode and a control electrode, means for supplying to said control electrode a succession of voltage pulses arranged recurrently to render said discharge device conductive for predetermined pulse intervals, a capacitive pulse-forming energy storage element, a pulse load circuit connected to said pulse-forming element through said discharge device, said load circuit including an autotransformer and a unilateral conducting device in parallel circuit relation, and means for suppressing parasitic electric oscillations in said inductive element at the termination of each pulse discharge of said pulse-forming element comprising a non-inductive resistor and capacitor in series connected in parallel circuit relation with the primary winding of said autotransformer,

7. In combination, an electron discharge device including an anode, a cathode and a control electrode, means for supplying to said control electrode a succession of voltage pulses arranged recurrently to render said discharge device conductive for predetermined pulse intervals, a capacitive pulse-forming energy storage element, a load circuit including an autotransformer having a primary winding connected across said capacitive pulse-forming element through said discharge device and a unilateral conducting device connected to the secondary winding of said transformer, and means for suppressing electric voltage oscillations across said transformer at the termination of each pulse discharge of said capacitive element comprising a non-inductive resistor and a blocking capacitor connected in series circuit relation across the terminals of said transformer primary winding.

DONALD E. MAXWELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,059,219 Farnsworth et al. Nov. 3, 1936 2,074,496 Vance Mar. 23, 1937 2,144,351 Vance Jan. 17, 1939 2,149,077 Vance Feb. 28, 1939 2,228,821 Hansen Jan. 14, 1941 2,294,388 Dawson Sept. 1, 1942 2,309,672 Schade Feb. 2, 1943 FOREIGN PATENTS Number Country Date 497,147 Great Britain Dec. 9, 1938 

